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<title>Which browsers can handle 'Content-Encoding:'?</title>
<meta name="author" content="Michael Schröpl" />
<meta name="description" content="A description of the behaviour of the most popular web browsers regarding compressed page contents" />
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<a title="mod_gzip - what's that, anyway?" href="index.htm">mod_gzip</a>
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<p>Browsers</p>
<a title="How do Firewalls handle 'Content-Encoding:'?" href="firewalls.htm">Firewalls</a>
<a title="An example configuration for mod_gzip" href="config.htm">Configuration</a>
<a title="Complete description of mod_gzip status codes" href="status.htm">Status Codes</a>
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<div id="content">
<h1>Which browsers can handle <code>Content-Encoding: gzip</code> ?</h1>
<h2><a id="Netscape3"></a>Netscape 3</h2>
<p>This browser uses HTTP/1.0. It doesn't send an <code>Accept-Encoding</code> header, thus doesn't request compressed content from a server.</p>
<p>The browser <strong>does not yet support the processing of compressed page content</strong>. If it receives gzip compressed content, it recognizes that there is an encoding <code>gzip</code> unknown to it <small>(and displays a corresponding message to the user)</small>, but after that it displays the compressed page content within the browser windows. Serving compressed content unconditionally <small>(like in statically precompressed documents)</small> this browser isn't good for.</p>
<p>A web server correctly evaluating the <code>Accept-Encoding</code> header is able to serve usable, uncompressed data to the browser.</p>
<h2><a id="Netscape4"></a>Netscape 4</h2>
<p>This browser uses HTTP/1.0. From version 4.06 on it is sending the header <code>Accept-Encoding: gzip</code>.</p>
<p>Nevertheless the implementation of handling compressed content contains a number of <strong>bad errors</strong>:</p>
<ul>
<li>In many 4.x versions <strong>Cascading Style Sheets</strong> that are loaded from a separate file by use of the HTML tag<br />
<code><link rel="stylesheet" type="text/css" href="</code><i>file name</i><code>" /></code><br />
will <em>not</em> be decompressed and thus not evaluated.</li>
<li>In many 4.x versions <strong>JavaScript code</strong> that is loaded from a separate file by use of the HTML tag<br />
<code><script type="text/javascript" src="</code><i>file name</i><code>" /></code><br />
will be decompressed and evaluated <em>only</em> if a <strong>cache is activated</strong> in the browser.<br />
<small>(In case of activated JavaScript in the browser this problem may additionally cause JavaScript error messages if the HTML source code references definitions of the included JavaScript code.)</small></li>
<li>In many 4.x versions the HTML code of the document will be decompressed before <strong>printing the page content</strong> <em>only</em> if a <strong>cache is activated</strong> in the browser; the same applies to the <strong>display of the page content in print preview mode</strong>.</li>
<li>In many 4.x versions the <strong>display of HTML source code</strong> of the current documents works correctly <em>only</em> if a <strong>cache is activated</strong> in the browser. Otherwise an empty screen will be displayed instead of the source code.</li>
<li>In many 4.x versions the correct handling of <strong>gzip compressed images</strong> may fail in the browser, under circumstances that don't seem to be easily reproducable. <small>(Anyway, images should be optimized manually by using chosing the correct file type, using a minimal color depth for GIF and the like - gzipping an already compressed file format won't lead to an additional gain worth mentioning.)</small></li>
<li>In the versions 4.06 to 4.08 there are even problems with the processing of JavaScript code inside the <code><head></code> section of compressed HTML pages.</li>
</ul>
<p>In all these cases Netscape 4 seems to use the received, yet uncompressed version; obviously the invocation of a <small>(surely available)</small> function for content compression simply has been forgotten at the decisive positions.</p>
<p>And some part of the errors described above may only be in effect if the browser's cache has been set to 0 MB and/or switched off - so it seems Netscape 4 is somehow using the browser cache as temporary storage for decompressing ...</p>
<h2><a id="Mozilla"></a>Netscape 6 & 7 and Mozilla 0.9.x & 1.x</h2>
<p>This browser uses HTTP/1.1. From Netscape 6.2 on <small>(= Mozilla 0.9.4)</small>, it is sending the header <code>Accept-Encoding: gzip, deflate, compress;q=0.9</code>.</p>
<p>Mozilla 0.9.9+ und Netscape 7.0PR1 allow the user to define the HTTP version as well as the content of this headers in its configuration; in Mozilla 1.1alpha this function is no longer visible in the preferences but can be defined in the configuration file <code>defaults/pref/all.js</code> <small>(under the name <code>network.http.accept-encoding</code>)</small>.</p>
<p>Processing compressed content works <em>if the browser has requested compressed content</em>; otherwise it ignores the HTTP header <code>Content-Encoding: gzip</code> although it would be able to decompress the content.</p>
<h2><a id="Microsoft"></a>Microsoft Internet Explorer 4.0, 5.0, 5.5 and 6.0</h2>
<p>This browser uses either HTTP/1.0 or HTTP/1.1, depending upon the settings in the internet options. It is sending the header <code>Accept-Encoding: gzip, deflate</code> - but only if using HTTP/1.1.</p>
<p>Processing compressed content works <em>if the browser has requested compressed content</em>; otherwise it ignores the HTTP header <code>Content-Encoding: gzip</code> although it would be able to decompress the content.</p>
<p>Some older versions of the Internet Explorer seem to fire the JavaScript event <code>onLoad</code> already when they have successfully <em>received</em> a JavaScript file referenced within the <code><head></code> section instead of waiting for the successful completion of <em>decompressing</em> its content.</p>
<h2><a id="Opera-old"></a>Opera 3 and 4</h2>
<p>Opera 3.5 does <em>not yet</em> understand compressed content.</p>
<p>From version 4 on Opera is using HTTP/1.1. Opera 4.0beta3 did already try to communicate in compressed form but sent the header <code>TE: deflate, gzip, x-gzip, identity, trailers</code>, i. e. it expected gzip compression to be applied as <em>Transfer Encoding</em>, not as <em>Content Encoding</em>.</p>
<h2><a id="Opera"></a>Opera 5 and 6</h2>
<p>Starting with version 5.12 <small>(or a bit earlier)</small> Opera is sending the header <code>Accept-Encoding: deflate, gzip, x-gzip, identity, *;q=0</code>. Processing compressed content works without known problems in versions 5 and up.</p>
<p>Opera 6 <small>(as the single known browser up to now)</small> decompresses gzip-compressed document content even if the server did <em>not</em> call its attention to it by serving the HTTP header <code>Content-Encoding: gzip</code>.</p>
<h2><a id="Lynx"></a>Lynx</h2>
<p>Lynx supports gzip-compressed communikation since version 2.6.</p>
<p>The first Lynx versions had to start a separate process using an external <code>gzip -d</code> command for decompressing. This of course leads into problems in case of no <code>gzip</code> command being available.</p>
<p>More recent versions of Lynx <small>(since 1997-08-14, see Lynx 2.8.0 <code>CHANGES</code> file)</small> use the <code>zlib</code> library for decompressing and don't have these problems any more.</p>
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<a href="http://validator.w3.org/check/referer"><img alt="" title="valid XHTML 1.1" height="31" width="88" src="valid-xhtml11.png" /></a><a href="http://jigsaw.w3.org/css-validator/check/referer"><img alt="" title="valid CSS" height="31" width="88" src="valid-css.png" /></a>
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<p id="mail">(<a href="mailto:michael.schroepl@gmx.de?subject=mod_gzip">Michael Schröpl</a>, 2002-09-16)</p>
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